The present disclosure and invention relate to methods for the preparation of acetic acid via the oxidation of ethane, including the preparation of high purity acetic acid that comprises very low concentrations of formic acid impurity.
Ethane, C2H6, a colorless and odorless hydrocarbon gas at room temperature, is common in nature and can be isolated on an industrial scale during the purification of natural gas or as a byproduct of various petroleum refining processes, such as from catalytic cracking of higher hydrocarbons. The value and/or price of ethane is, however, currently relatively low, either as a fuel, or as a potential feedstock for making other petrochemicals such as ethylene, or acetic acid.
Acetic acid, CH3—CO2H, is a worldwide commodity produced at the level of hundreds of millions of tons per year, as a precursor for vinyl acetate monomer and polymers, and for making various acetate ester solvents and other chemicals. Although many processes for making acetic acid have been developed and commercially employed over the years, acetic acid production by methanol carboxylation is currently dominant in the chemicals industry. In those processes, methane or coal are converted to syngas (mixtures of hydrogen and carbon monoxide), then the syngas is catalytically converted to methanol, which is then reacted with carbon monoxide (separated from syngas) in the presence of rhodium or iridium iodide catalysts, to produce acetic acid. Although modern methanol carboxylation processes are quite efficient, they also require multiple steps and the use of high cost corrosion resistant materials of construction. Therefore, simpler and less capital intensive methods for making acetic acid remain of potential interest to industry, especially since efficient catalysts for hydrogenating acetic acid to make ethanol for use as an automotive fuel component have been discovered recently, so that the continuing expansion of the markets for acetic acid is expected in the long run.
While ethane oxidation processes produce fewer kinds and quantities of impurities heavier than ethanol, ethane oxidation processes also tend to produce relatively high quantities of formic acid, which because of its chemical and physical properties that are similar to acetic acid, is very difficult to separate from acetic acid, particularly to produce very high purity acetic acid containing very low levels of formic acid. Even low levels of formic acid in acetic acid are undesirable because the formic acid promotes corrosion of metal vessels used to make and store acetic acid.
Accordingly, there remains a need for new and less expensive methods for making high purity acetic acid. Applicants have developed such new methods, via application of the techniques of crystallization for purifying acetic acid made by the oxidation of ethane.